1. Field of the Invention
The present invention relates to a valve unit, and more particularly to a valve unit having an ON-OFF switch constituted by a valve and an associated valve seat.
2. Description of the Prior Art
In the prior art, to obtain an electric signal indicating the open/close state of a valve unit, valve units in which an ON-OFF switch is constituted by a valve and the associated valve seat are widely used. Such a valve unit is needed, for example, for constituting a valve unit driving circuit in which the driving pulse applied to the valve unit is corrected in response to the timing of opening/closing of the valve unit in order to make the open/close timing of the valve unit coincide with a target timing. It is also needed for constituting a fuel injection valve which is capable of producing an electric signal indicating the injection timing of fuel.
As such a valve unit, there is disclosed for example in U.S. Pat. No. 4,111,178 (corresponding to DE-OS No. 2748447) a fuel injection valve in which a mechanical switch is constituted by a needle valve and a nozzle body in order to obtain an electric signal indicating the timing of the beginning of fuel injection and the timing of the end of fuel injection in response to the movement of the needle valve. In the disclosed fuel injection valve, a nozzle body and a needle valve smoothly moving in the guide hole of the nozzle body are formed of an electrically conductive material and the outer surface of the needle valve is covered with a ceramic insulation film of a thickness between approximately 0.2 .mu.m and 0.3 .mu.m, or an insulation film formed by the sputtering of aluminum oxide.
However, when the ceramic thin film is used as the insulation film the durability is insufficient and when the insulation film formed by the sputtering of aluminum oxide is used the insulation film is liable to peel off from the outer surface of the needle valve. In either case, consequently, stable use over long periods is impossible.